Process for improving the melt flow index of thermoplastic polymers

ABSTRACT

A novel process for providing a thermoplastic polymer having unique thermal characteristics including the step of cooling the polymer during extended mastication until the melt index of the polymer is substantially increased.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 616,053filed June 1, 1984, now U.S. Pat. No. 4,632,977.

TECHNICAL FIELD

The present invention relates generally to endodontia methodologies andprocedures. More particularly, the present invention relates to the useof thermoplastic polymers in endodontia methodologies and procedures.Specifically, the present invention relates to a process for producingthermoplastic polymers to provide a thermoplastic polymer having a muchimproved melt flow index so that it is particularly suited for theobturation of root canals.

BACKGROUND ART

Endodontia is a branch of dentistry specializing in diseases of thetooth pulp. A primary corrective procedure for diseased pulp is toremove it, clean out and shape the resulting root canal and thenobturate the root canal space. The obturation procedure is criticalbecause if the apical seal is not adequate, the tissues underlying theroot canal may be exposed to foreign, deleterious matter.

Obturation of the root canal system typically involves the insertion ofa material within the root canal, which material must sealably adhere tothe dentin walls. Of particular concern is a fluid-tight seal betweenthe inserted material and the apical foramen region. Additionally,endodontic therapy requires that the inserted material conform, andeffect a seal, to the dentin wall irregularities as well as any lateralcanals. Various methodologies for obturating root canals have beendeveloped and are disclosed in the prior art. A thorough understandingof these procedures is necessary to appreciate the significance andnovelty of the present invention.

While many types of obturating material have been used, modernendodontia techniques utilize trans-polyisoprenes such as gutta-perchaand balata or other species thereof. In order to avoid a repetitivelydetailed identification of the trans-polyisoprenes utilized, thespecification will hereinafter simply refer to gutta-percha.Gutta-percha, chemically, is the trans- isomer of natural rubber and isa tough, crystalline, thermoplastic polymer. The thermoplasticcharacteristic of gutta-percha makes it a particularly useful endodontiamaterial, because in the plasticized state gutta-percha readily adaptsto the contour of the dentin walls and tends to retain its shape aftercooling, albeit with some shrinkage.

Verification of the effectiveness of a particular obturation techniquemay be performed in vitro using dye penetrant, radiotracer penetrant,microscopic examination, sectioning, X-ray analysis or scanning electronmicroscope analysis, among others. Though in vivo assessment ispossible, a much greater period of time is required before the resultsare available. The particular analysis technique utilized will, in part,be determined by which aspect of the obturation seal may be of interest,as well known and understood by those skilled in the art.

One obturation technique involves the use of gutta-percha cones, orpoints, usually made in standard sizes, a plurality of which are fittedinto the root canal which has itself been generally conically shaped tofacilitate acceptance of the cones. After the canal has been cleaned andshaped, the dentin walls are typically coated with a sealer and a pointis inserted. The inserted point is then condensed with finger pluggersto force the tip of the point into conformation with the apical regionof the root canal. With this technique the gutta-percha point typicallyis not heated, and a second step, lateral condensation, is required.Lateral condensation is effected by inserting additional points into thecanal and compacting them with heated finger pluggers to force the pointmaterial into lateral conformity with the canal wall, and hopefully toconform the point material not only to the irregularities therein butalso to any laterally extending canals. Numerous difficulties can arisewith this method. A major inconvenience is having to impart a particularshape to the apex of the canal in order that it can accept a standardgutta-percha point. Irregularities in the dentin walls may allow for aless than satisfactory adaptation of the point to the canal cavity.Also, the sealer may not be uniformly distributed within the interfacebetween the point and the dentin walls.

Another obturation technique is generally referred to as verticalcondensation and utilizes heated gutta-percha. The canal is prepared anda sealer introduced, as previously described. The gutta-percha to beintroduced into the canal, however, is first heated. A plurality ofwarmed gutta-percha segments may be compactably inserted into the canalchamber. Heating the gutta-percha reduces its viscosity and therebyallows the material more readily to adapt to the dentin walls thanunheated gutta-percha. Unexpectedly, however, the degree of lateralcondensation, or conformity, has been found to be reduced.

When employing this technique, it has been found that the materialextruded into lateral canals consists primarily of sealer. Asanticipated, fillings made by vertical condensation techniques showclose adaptation of the gutta-percha to the dentin walls. However, voidsare often seen, possibly due to reduced lateral condensation, and theroot canal sealer is not always present at the interface. Thegutta-percha also tends to cool quickly, particularly during theconsiderable time required for the vertical condensation technique. Oncethe material cools, viscosity increases and lateral flow issubstantially reduced. Also, seams have been observed which may beindicative of incomplete conjoining of different gutta-percha segments.

A third technique uses a solvent such as chloroform to soften thegutta-percha. While this technique shows good adaptation in the apicalregion, voids are often seen coronal to this section. The surface of thefilling is often wrinkled which is most probably due to shrinkage of thematerial. Such shrinkage is an undesirable feature as it may reduce theintegrity of the required seal between the filling material and theobturated canal. Shrinkage may also occur in the vertical condensationtechnique during cooling.

The use of plasticizers or other additives in the nature of processingaids has been considered, but there are three major drawbacks. First, itwould require lengthy governmental testing and subsequent approval toassure that the processing additives are not deleterious for use inendodontic therapy. Second, even though such approval might be acquiredthere would be a reluctance on the part of the majority of practitionersto accept a new product without considerable clinical evidence of itssatisfactory performance without incident. Third, processing additivesappear to create unfavorable shrinkage characteristics which would workto destroy the seal required to obturate a root canal.

Although the above obturation techniques--with perhaps the notedexception as to the use of additives--generally provide reasonablyacceptable results when performed properly, the methods are timeconsuming, and, therefore, costly, and require considerable care toeffectuate a fluid-tight seal, particularly at the apical foramen.

One of the most promising obturation techniques is generally referred toas thermoplastic injection of a polymeric material such as gutta-percha.The important feature of this technique is that the polymer is heated toits molten, or plasticized, state--typically about +160° C. The polymeris then forced, under mechanically generated pressure, into the rootcanal system.

One of the earlier methods for in vitro "thermoplastic injection" isdescribed by Yee, et al, Three-dimensional Obturation of the Root CanalUsing Injection-Molded, Thermoplasticized Dental Gutta-Percha, JOURNALOF ENDODONTICS, Vol. 3, No. 5, May, 1977. The root canal system isshaped and cleaned using conventional techniques. The gutta-percha isnext introduced into the root canal cavity using an endodontic pressuresyringe. An 18-gauge needle may typically be used inasmuch as it is themaximum size that fits conveniently into the root canal in humananterior teeth.

To prepare for the injection technique, gutta-percha cones are manuallyloaded into a syringe, and the barrel, with the needle attached, is thenheated in a glycerine bath until an unrestrained flow can be achieved.This occurs at about +160° C. The needle is then inserted into thecavity and the gutta-percha extruded therethrough to fill the cavity.When a slight resistance from the injected material is felt, the needleis withdrawn a few millimeters coronally and more material is extruded.This process continues until the canal is completely obturated.

Analysis of the results of obturation by this technique shows few voidsand excellent adaptation of the polymer to the dentin walls. Aparticular advantage is that the plasticized polymer flows bothvertically and laterally. Moreover, the sealer tends to be evenlydistributed and lateral canals can be effectively obturated.

While the above-described method is useful for in vitro analysis ofinjection-molded thermoplasticized polymers, clinical application isseverely limited due to the excessively high temperatures required toplasticize gutta-percha and similar polymers. These temperatures makehandling of the applicator and working in the periapical and oralregions somewhat difficult and possibly unacceptable because of the fearin the mind of the clinician that its use could, even remotely, beinjurious to the patient.

In order to circumvent the high temperature related problems which makethe injection process clinically undesirable and impractical, much workhas been dedicated to developing more feasible delivery systems. Onesuch system is described by Marlin, et al, Clinical Use ofInjection-Molded Thermoplasticized Gutta Percha for Obturation of theRoot Canal System: A Preliminary Report, JOURNAL OF ENDODONTICS, Vol. 7,No. 6, June, 1981. The equipment includes an injection syringe and anelectrical heating unit. The barrel of the syringe carries an electricalheating element and is insulated not only to minimize heat dissipationbut also to protect the clinician and patient. The level of heat isvariable depending, in part, on the needle gauge. Standard gutta-perchapoints are loaded into the syringe, plasticized, and the gutta-percha isthen inserted in a manner similar to the Yee, et al in vitro techniquehereinbefore described.

While this method of delivery is an improvement on some aspects of priorknown systems, it does necessitate the use of a considerably morecomplex and expensive delivery system. It must be understood, however,that this method is only a delivery system; it is not a filling system.Tests have revealed that by this procedure the plasticized gutta-perchais delivered only one-half (1/2) the distance between the tip of theinjection needle and the apex of the canal. Further manipulation, aswith finger pluggers, is required to assure that the canal is completelyfilled. Moreover, the high temperatures at which the gutta-percha mustbe delivered by this procedure appears, itself, to engender two majordrawbacks. First, mental unacceptability--the clinician remains fearfulof injecting material at such high temperature into a human patient.Second, questionable operability--it appears that the high temperaturedifferential between the injected gutta-percha and its surroundingenvironment likely results in excessive shrinkage of the gutta-percha asit cools.

Another method is recounted by Lugassy, et al, Root Canal Obturationwith Gutta-Percha: A Scanning Electron Microscope Comparison of VerticalCompaction and Automated Thermatic Condensation, JOURNAL OF ENDODONTICS,Vol. 8, No. 3, March, 1982. The technique was developed by McSpadden andis generally referred to as automated thermatic condensation. Thistechnique uses a compactor, similar to a Hedstroem file, mounted on acontra-angle. The compactor plasticizes the gutta-percha within the rootcanal system and provides lateral as well as vertical compaction.

According to this technique, after the root canal is shaped andprepared, a compactor size is selected one size smaller than the largestreamer used near the apical constriction. A standard gutta-percha pointis inserted into the canal, and the compactor is then rotated at a speedof approximately 10,000-15,000 r.p.m. The direction of rotation must besuch as to assure an apical vector for the gutta-percha compaction.Essentially, the rotary tool provides frictional heat whereby thegutta-percha is plasticized and adequate lateral and verticalcondensation is achieved.

While the McSpadden technique is an improved method for avoiding theexcessive heat problem, the technique still requires sophisticatedhardware, consummate skill and strict adherence to specific guidelinesto obtain predicted results.

It is clear, therefore, that a viable need exists for an improvement inthe art of endodontia whereby thermoplastic polymers can be quickly andinexpensively injected with a minimal risk from excessive temperaturesand without elaborate delivery systems.

DISCLOSURE OF THE INVENTION

It is, therefore, a primary object of the present invention to providean improved process for preparing a material for use in root canalobturation.

It is another object of the invention to provide an obturating materialwhich plasticizes at a clinically feasible temperature and which can bedelivered to fill a root canal by virtue of a standard syringe.

It is still another object of the invention to provide a novelimprovement to a method of obturating root canals.

It is a further object of the invention to provide a process by whichthe melt index of a thermoplastic polymer can be substantiallyincreased.

It is yet another object of the present invention to provide an improvedprocess for preparing a thermoplastic material so that the materialexhibits minimal shrinkage during cooling.

These and other objects of the invention, as well as the advantagesthereof over existing and prior art forms, which will be apparent inview of the following specification, are accomplished by meanshereinafter described and claimed.

In general, the present invention relates to a process forplasticization of thermoplastic polymers selected from the groupconsisting of gutta-percha, balata and synthetic trans-polyisoprenehaving an unprocessed melt flow index falling within the range ofapproximately 0.2 to 0.8 grams per 10 minutes comprising the steps ofmasticating the polymer with sufficient shear to heat the polymer,cooling the polymer to about 60° C. while continuing to masticate and,discontinuing the mastication of the polymer when the melt index reachesthe desired level.

A related process for the plasticization of thermoplastic polymers ofthe foregoing type comprises the steps of masticating the polymer withsufficient shear to heat the polymer to a range of from about 90° to110° C., cooling the polymer and monitoring the temperature thereof asit is masticated and, discontinuing the masticating step when thetemperature of the polymer drops to a range of from about 60° to 72° C.

A related process for the plasticization of the foregoing thermoplasticpolymers comprises the step of masticating the polymer with sufficientshear to create heat, cooling the polymer and continuing the masticationfor at least about 15 hours and, thereafter discontinuing themastication.

The application of the aforesaid processes to thermoplastic polymersselected from the group consisting of natural and synthetic trans-polyisoprenes results in a product having novel characteristicsincluding a melt flow index exceeding at least about 10 grams per 10minutes and which can reach 500 grams per 10 minutes or higher. As such,these polymers are particularly suited for the obturation of rootcanals. One unique property is an extremely high melt flow index despitethe fact that plasticizers, chemical processing aids and solvents arenot employed. Typical examples of the trans-species include gutta-perchaand balata, although the present invention is not limited to these. Asnoted hereinabove, reference has been made throughout the specificationto gutta-percha as a matter of convenience and because it has beenexemplified hereinbelow. Nevertheless, it is to be understood thatpractice of the present invention is more broadly directed toward theforegoing trans- polyisoprenes.

When these materials are processed it has been found that the heatproduced by mastication raises the temperature of the polymer beingmasticated to approximately the upper reach of the range ofapproximately 90° to 110° C. The temperature will slowly decrease duringcontinued mastication, while remaining within the stated range, untilsuch time as the rate of change of the melt flow index undergoes aquantum change. However, the process of the present invention employs anovel cooling step during mastication which reduces the time somewhatfor the temperature reduction to occur. Generally coincident with thequantum change in the rate of change of the melt flow index, ashereinafter more fully explained, the temperature will have reducedapproximately twenty to forty-five percent (20-45%) from its highestreading. This brings the temperature of the material being masticateddown to the range of 60° to 72° C.

Embodiments of the processes for producing the novel polymerincorporating the concepts of the present invention are disclosed hereinby way of example without attempting to disclose all of the variousforms and modifications in which the invention may be carried out; theinvention being measured by the appended claims and not by the detailsof the specification.

BRIEF DESCRIPTION OF THE DRAWING

The drawing FIGURE constitutes a graph in which the most criticalquality of the novel polymer--the melt flow index--is plotted againsttime of mastication.

PREFERRED EMBODIMENT OF THE INVENTION

The plasticization of the thermoplastic polymer according to the conceptof the present invention provides a polymer having novel characteristicsthat are particularly suited for obturating root canals by injectiontechniques. Specifically, the plasticized polymer so processed will flowsufficiently freely from a needle to fill the full apical cavity andflow laterally to fill any and all irregularities within the canal, andgenerally even including lateral canals. Moreover, such materials havebeen found to perfect an effective seal to the dentin walls such that asealer need not even be employed.

One suitable parameter by which to measure the relative acceptability ofa polymer for use as a material by which to fill a root canal is itsviscosity. A standardized scale by which to characterize the viscosityof a thermoplastic polymer is its melt flow index.

The melt index is the amount, in grams, of a thermoplastic resin whichcan be forced through an orifice of 0.0825 inch diameter (2.09 mm) whensubjected to a force of 2160 grams in 10 minutes at +105° C. Polymerspossessing higher melt flow indices--i.e., on the order of two to threeorders of magnitude greater than the normal 0.2 gms/10 min--readilyconform to the cavity into which they are injected, and if the melt flowindex could be maintained in such an elevated state during the time ittakes to perform the filling procedure, the polymer would flowsufficiently to fill the full extent of the canal. Heretofore asufficiently high melt flow index could be achieved only by elevatingthe temperatures of the polymer, but unfortunately, the sufficientlyhigher melt flow index could not be maintained for a period of time toachieve a satisfactory filling of the canal solely by virtue of the flowof the polymer.

According to the concept of the present invention the melt flow index ofa thermoplastic polymer such as gutta-percha, balata and the like can beradically and unexpectedly increased by masticating the material for amarkedly extended period of time. The masticating operation may beperformed with any conventional milling or mixing apparatus, eitherinternal or external, as more fully hereinafter discussed, but theduration of the masticating process is uniquely and substantiallymodified in a novel way and clearly distinguishable from conventionalmilling processes.

Milling is one of the well-known processes for masticating, and/ormixing, viscous and elastic materials. A typical two-roll, open mill--asused for mixing, warm-up, feeding and cracking in the rubberindustry--may be employed. Such a mill includes two surface rolls, sethorizontally close together. The stock material is forced between therolls, which run at different speeds and form a band of material aboutat least one of the rolls.

Mastication can also be provided in an internal mixer such as a doublearm mixer which employs a pair of blades which communicate with eachother as well as the internal walls of the mixer. Such apparatus can beheated or cooled externally as a means of controlling the temperature ofthe material within.

Whether milling, mixing or other apparatus is employed, the shearingaction of the mastication process generates considerable heat. Moreover,while a typical mixing operation on a mill may require only a fewminutes, the processing of a polymer according to the concept of thepresent invention takes a considerably extended period of time, ashereinafter more fully explained.

Although the preferred embodiment is hereafter described withparticularity in reference to gutta-percha and/or balata, this is to beconstrued as exemplary only, and not intended to be, in any sense,limiting.

When polymers such as gutta-percha or balata are prepared as the stockmaterial in making "points", cones and the like for endodontic therapy,numerous fillers may be added to the gutta-percha or balata, includingbarium sulfate, zinc oxide or titanium oxide. The final compoundtypically is mixed in a conventional milling operation for approximately20 minutes to an hour.

However, according to the concept of the present invention the polymeris extensively masticated prior to intermixing the fillers normallyemployed. It should be appreciated that the fillers could well beintroduced prior to mastication, but inasmuch as the final product willcontain on the order of 19-21 percent gutta-percha, it is more efficientto process the gutta-percha prior to intermixing the standard fillers.Moreover, it has also been found that the process of increasing the meltflow index is facilitated by cooling the polymer as it is masticated.

By way of example, unprocessed gutta-percha has a melt flow index ofapproximately 0.2 grams per 10 minutes. The normal milling time tointermix the various fillers (20 to 60 minutes) leaves the melt flowindex virtually unchanged. In fact, during the first several hours ofmastication the melt flow index shows a very modest increase. However,at some point between approximately the 10th and 20th hour there is atotally unexpected quantum increase to the rate of change of the meltflow index per hour of mastication such as continued mastication for anadditional, comparable period of time effects change in the melt flowindex on the order of at least two orders of magnitude.

Shearing of the polymer during masticating will cause the temperature ofthe polymer being masticated to rise initially to a range of from about90° to 110° C. This appears to be an effective temperature for theinitial mastication of gutta-percha in a closed internal mixer as wellas an open mill.

Although heat is therefore not required during mastication, the mixingapparatus can be warmed initially to bring the temperature of thepolymer from ambient up to a suitable mixing or milling temperature atwhich the polymer mass is sheared. Such a suitable temperature would beup to about 90° to 110° C., the temperature that results from shearingand can eliminate 30 to 45 minutes of mastication time that is normallyrequired for this temperature to be obtained solely by mastication.

Once this range is reached, the process of the present invention callsfor a cooling step that not only inhibits further temperature increasebut also aids in reducing the temperature of the polymer. When the meltflow index has been markedly increased, the temperature induced by theshear of mastication is greatly reduced. In fact, the temperature of thegutta-percha has been found to display an overall decrease on the orderof 20% to 45% at or shortly after the aforedescribed quantum increase inthe rate of change of the melt flow index--even as the masticationprocess is continued. Specifically, the temperature of the gutta-perchais reduced to the range of from about 60° to 72° C.

Although this temperature reduction occurs in the polymer when the meltflow index has been greatly increased, it has now been found that a stepof cooling the polymer during mastication will decrease the time ofmastication. The present invention is not limited to any particularapparatus or process whereby cooling is imparted as those skilled in theart will appreciate and be able to select equipment compatible withtheir mastication apparatus. Generally, the temperature of the polymershould be maintained at about 60° C. and can vary by at least ±10° C.

While care must be taken not to cool the polymer to such a degree thatit no longer sticks to the shearing elements, i.e., blades or rolls, ofthe mastication apparatus, there is still a relatively wide range oftemperature over which the mastication step can be conducted as comparedto the range that would exist during most of the mastication processwithout cooling--about 90° to 110° C. It has now been found that theduration of the mastication step, once the polymer has reached itsshearing temperature, is shortened by about two hours or more if thepolymer is cooled as noted hereinabove. However, a point is reachedwhere too much cooling will again lengthen the mastication step andtherefore, the operator can expect optimum efficiency at a temperatureclose to 60° C. By shearing temperature, is meant the temperature thatis imparted to the polymer solely by the mechanical forces of shearingin the apparatus.

In a similar vein, the mastication step is conducted for at least about10 hours or until the desired level of melt flow index is obtained,i.e., the level at which a root canal can successfully be obturated. Asnoted hereinabove, the initial melt flow index of these polymers is onthe order of 0.2 to 0.8 grams per 10 minutes and conventionalprocessing, relying on chemical additives has provided only about 10grams per 10 minutes. The majority of dental practitioners areaccustomed to using a product having this melt flow index as the designof existing equipment and root canal techniques will attest. However,practice of the present invention will provide polymers having melt flowindices of 20 grams, 50 grams, 100 grams, 250 grams, even 500 grams per10 minutes and all ranges therebetween.

The desired level of melt flow index is a function of many factorsincluding the anatomy of the root canal; the equipment that is selectedand the desired techniques of the clinician. A primary objective is thatthe polymer be deliverable in a flowable state which requires heat.While a polymer having a melt flow index of 500 will flow at a lowertemperature than one where the melt flow index is 50, the clinician mayprefer to use a higher temperature, or to alter the flow characteristicsof the polymer. If the technique involves condensing, to move thematerial into the canal laterally, a lower melt flow index or lessflowable material could be selected.

Thus, it should be understood that a variety of melt flow indices can beuseful and that the mastication time is a function of the melt flowindex obtained. Generally, after about 15 hours the melt flow index willcontinue to increase dramatically with time such that a melt flow indexof about 500 grams/10 minutes will be obtained at about 30 hours. Whilegreater values can be obtained, the usefulness of such products forobturating root canals may be minimal. Nevertheless, such a productcould have other uses and therefore, the present invention should not belimited to 30 hours or melt flow indices of 500 grams/10 minutes asother levels may be desired and can be obtained following theseteachings.

In order to quantify this wholly unexpected result, several 1100-1600gram batches of unprocessed gutta-percha were continuously masticated inan internal mixer for varying periods of time. The temperature of thegutta-percha was noted at various intervals and a sample of thegutta-percha was then taken and tested by the standard proceduredescribed above to determine the melt flow index. Those results are setforth below in Tables I-III.

In Table I, melt flow indices for three samples of gutta-percha, ExampleNos. 1-3, were determined with polymer mixing begun without any initialheating to bring the polymer to 94° C. Once this temperature wasobtained by shearing action, cold water circulation about the exteriorof the apparatus was begun to chill the stock during the successivehours of mastication. In each instance, approximately 30 minutes ofinitial mastication was required to take the polymer from ambient toabout 94° C.

In Table II, melt flow indices for two additional samples ofgutta-percha, Examples Nos. 4-5, were determined for polymers heated atthe beginning of the mastication to bring their temperature quickly toabout 91° C. after which cooling followed. In both instances thetemperature was obtained in about 15 minutes.

In Table III, Example Nos. 6-8 are presented. Each polymer was subjectedto a heat start as explained for Table II of about 15 minutes.Subsequent mixing time with chilling is reported as are melt flowindices of approximately 50, 100 and 250 grams/10 minutes.

                  TABLE I                                                         ______________________________________                                        Melt Flow Index and Polymer Temperature                                       During Mastication Without Initial Heating                                    Time           Temperature                                                                              Melt Flow Index                                     (Hours)        °C. (Gms/10 min)                                        ______________________________________                                        Example 0          94         0.2                                             No. 1   .08        94         --                                                      .17        94         --                                                      .25        94         --                                                      .33        94         --                                                      .42        94         --                                                      .50        99         --                                                      1.0        96         --                                                      1.5        93         --                                                      2.0        88         --                                                      2.5        86         --                                                      4.0        77         --                                                      5.0        72         --                                                      6.0        69         --                                                      7.0        65         4.0                                                     22.75      61         221.9                                                   25.25      60         272.7                                                   29.75      59         409.8                                                   31.5       59         470.0                                           Example 0                     0.2                                             No. 2   6.75       62         7.2                                                     23.5       62         170.5                                                   25.5       62         201.9                                                   27.0       62         227.4                                                   29.0       62         261.3                                                   30.5       61         283.6                                                   32.5       61         325.0                                                   37.25      61         443.8                                                   37.75      61         465.0                                           Example 0          92         0.2                                             No. 3   .08        92         --                                                      .17        92         --                                                      .25        92         --                                                      .33        92         --                                                      .42        92         --                                                      .50        102        --                                                      1.0        98         --                                                      1.5        98         --                                                      2.0        92         0.7                                                     2.5        88         --                                                      4.0        79         --                                                      5.0        75         2.6                                                     6.0        70         --                                                      7.0        65         8.4                                                     22.75      61         186.0                                                   26.0       60         223.2                                                   28.0       60         310.3                                                   30.5       59         387.1                                                   33.5       59         479.3                                           ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Melt Flow Index and Polymer Temperature                                       During Mastication With Initial Heating                                       Time           Temperature                                                                              Melt Flow Index                                     (Hours)        °C. (Gms/10 Mins)                                       ______________________________________                                        Example 0                     0.2                                             No. 4   .75        91         --                                                      2.0        91         --                                                      4.5        80         1.2                                                     6.0        77         --                                                      7.25       73         2.4                                                     10.0       71         6.0                                                     23.0       59         --                                                      23.25      59         133.91                                                  24.25      61         158.8                                                   26.0       61         203.1                                                   28.2       61         280.1                                                   30.0       62         345.4                                                   31.0       62         387.9                                                   31.7       62         409.8                                                   33.75      62         495.0                                           Example 0          91         0.2                                             No. 5   .75        91         --                                                      1.5        91         0.4                                                     3.5        81         1.2                                                     5.0        75         2.5                                                     6.5        70         4.5                                                     10.5       61         --                                                      22.0       60         246.9                                                   23.5       60         282.1                                                   25.0       60         335.7                                                   27.5       60         429.6                                                   29.0       60         507.7                                           ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Particular Melt Flow Index and Polymer Temperature                            During Mastication With Initial Heating                                       Time           Temperature                                                                              Melt Flow Index                                     (Hours)        °C. (Gms/10 Mins)                                       ______________________________________                                        Example 0                     0.2                                             No. 6   16.0       57         18.5                                                    17.0       57         25.2                                                    18.0       56         33.3                                                    19.0       54         47.5                                            Example 0                     0.2                                             No. 7   2.0        78         0.6                                                     6.0        69         1.7                                                     8.0        69         2.9                                                     23.5       69         106.2                                           Example 0                     0.2                                             No. 8   1.1        79         0.6                                                     4.1        71         2.6                                                     20.1       59         176.0                                                   21.1       59         150.0                                                   22.0       58         157.0                                                   23.0       58         182.0                                                   24.0       59         221.0                                                   25.0       60         240.0                                           ______________________________________                                    

With reference first to Tables I and II, the change in the melt flowindex observed during the first several hours would provide nomotivation to continue the mastication procedure. What is, therefore,unexpected is that by continuing for what might have been deemed aninordinate period of time one can perceive an increase in the melt flowindex from the approximate 0.2 gms/10 minute for unprocessedgutta-percha up to approximately 500 gms/10 minutes--a 2500 foldincrease.

Also by comparing the results of Table I with Tables II and III, it canbe seen that the time periods were generally about two hours shorter toobtain a melt flow index of 450 grams and above with the initial heatingstep than where it was omitted.

Mastication of Example No. 1 was begun at room temperature and raised to94° C. during shearing and then cooled over the next 31.5 hours. Coolingwas gradual for the first few hours as evidenced by the polymertemperatures. However, after 22 hours, the temperature of the polymerdropped several degrees, commensurate with the considerably increasedmelt index values. Generally, the same temperature profile was observedfor Examples No. 2 and 3, the differences being that a warmertemperature of water was employed which resulted in a lower melt indexat 22.75 hours than in Example 1 and a longer total mixing time. Example1 also reached a melt index of 470 after 31.5 hours while Example 3required 33.5 hours to reach 479 and Example 2 had only a melt index of465 after 37.75 hours.

Examples 4 and 5 in Table II were begun with heat as previously notedand it can be seen that the time was further shortened in Example 4 to29 hours to reach a melt flow index of 507. Finally, with respect to thedata in Table III, three different ranges of melt flow index have beenprovided which establish that desired values falling between the broadranges of 20 and 500 grams/10 minutes can be obtained controlling thetime of mastication.

With reference to the drawing figure, a plot of melt flow index versustime has been provided using values from Examples 1-8. Mastication forapproximately that period of time represented between points A and B onthe curve revealed the existence of a generally linear relationshipbetween the melt flow index and the time of mastication. Between pointsB and C a pronounced change occurred which is the "knee" of the curve,and it represents a transitional range heretofore designated as thequantum change. This knee was found to occur within a total range ofbetween 10 to 20 hours, with the majority of the samplings reflectingthe occurrence of the knee within an approximate range of from 14 to 19hours. It was also noted that the knee was fairly well developed by thetime the melt flow index reached approximately 20-25 grams/10 minutes,irrespective of the mastication time required to being the melt flowindex to that level.

Finally, from point C to, and past, point D the plot had a totallydifferent slope than encountered between points A and B. A generallylinear relationship was thus also determined to exist between the meltflow index and the time of mastication beyond the "knee."

Comparing the slope of the linear relationship between points A and Band the slope of the linear relationship between points C and D a changein excess of two orders of magnitude can be determined. Within the timeframe from points A to B, there is no indication that continuedmastication would produce the transitional rate of change represented bythe knee nor that the plot would thereafter slope more favorably toreflect markedly higher and higher melt flow indices with respect torelatively modest further mastication time.

Following processing, the customary additives, such as those identifiedabove, can be intermixed into the gutta-percha by a 20 to 60 minutemilling or other mixing process. The additives will tend to reduce themelt flow index on the order of at least 50% but this decrease can beobviated by additional mastication. Those skilled in the art willreadily appreciate that various means of providing additionalmastication can be employed.

Gutta-percha having such a markedly increased melt flow index may beheated to approximately 70° C. and injected through a needle fully tofill the entire cavity of a root canal. The gutta-percha so processedflows the full distance from the needle to the apex of the canal.Moreover, gutta-percha so processed will flow fully to the apex andlaterally into any and all irregularities, usually including lateralcanals, all without requiring additional manipulation with fingerpluggers or the like.

Gutta-percha with such an increased melt flow index also appears moreeffectively to "wet" the dentin wall and seal the same without thenecessity of employing a separate sealer. Finally, the gutta-percha soprocessed does not as readily pull away from the walls as a result ofshrinkage as it cools, thus maintaining the necessary seal.

The significance of this novel process is critical in fields such asendodontia. By increasing the melt index of the polymer, thermoplasticinjection molding can be performed at much lower and safer temperatures,such as about 66° C. Such lower plasticization temperatures makeclinical application of this technique readily available and obviate theneed for sophisticated delivery systems or alternative obturationtechniques. By processing the gutta-percha in accord with the novelprocess disclosed herein, the polymer can be plasticized at a clinicallymore acceptable temperature. The lower temperature also minimizesshrinkage of the material upon cooling, thus improving the integrity ofthe seal. It will also be appreciated by one skilled in the art that useof a higher melt-index polymer with other obturation techniques such as,for example, lateral and vertical condensation and automated thermaticcondensation, will greatly enhance the utility of those techniques.

The exact mixing or milling criteria will, of course, be dependent onthe particular flow characteristic desired for the material and theparticular use to be made thereof. Inasmuch as the present invention issubject to many variations and changes in detail, a number of which havebeen expressly stated herein, it is intended that all matter describedthroughout this entire specification be interpreted as illustrativerather than limiting.

It must be appreciated that for the endodontic procedures presentlycontemplated a melt flow index of 500 grams per 10 minutes has beenfound to be perfectly adequate. As a result, no attempts have been madeto continue mastication beyond that point. It should also be understoodthat there have been no indications that would dissuade one frombelieving that even higher melt flow indices would be obtained byapproximately increased mastication times.

It should thus be evident that a material prepared according to theconcepts of the present invention, or reasonably equivalent methods,will accomplish the objects of the present invention and otherwisesubstantially improve the art of thermoplastic polymers.

I claim:
 1. A process for the plasticization of thermoplastic polymerselected from the group consisting of natural and synthetictrans-polyisoprene having an unprocessed melt flow index faling withinthe range of approximately 0.2 to 0.8 grams per 10 minutes comprisingthe steps of:masticating said polymer with sufficient shear to heat thepolymer until the temperature reaches a range of from about 90° to 110°C.; cooling the polymer to about 54° C. while continuing mastication;and discontinuing the mastication of said polymer when the melt indexreaches the desired level.
 2. A process, as set forth in claim 1,wherein said thermoplastic polymer is selected from the group consistingof gutta-percha and balata.
 3. A process, as set forth in claim 1,including the additional steps of:heating the polymer during said firststep of masticating until the temperature reaches a range of from about90° to 110° C.; and discontinuing said step of heating when thetemperature reaches said range of about 90° to 110° C.
 4. A process, asset forth in claim 1, wherein mastication is discontinued when the meltflow index exceeds approximately 20 grams per 10 minutes.
 5. A process,as set forth in claim 1, wherein mastication is discontinued when themelt flow index exceeds approximately 50 grams per 10 minutes.
 6. Aprocess, as set forth in claim 1, wherein mastication is discontinuedwhen the melt flow index exceeds approximately 100 grams per 10 minutes.7. A process, as set forth in claim 1, wherein mastication isdiscontinued when the melt flow index exceeds approximately 250 gramsper 10 minutes.
 8. A process, as set forth in claim 1, whereinmastication is discontinued when the melt flow index exceedsapproximately 500 grams per 10 minutes.
 9. A process for theplasticization of thermoplastic polymers selected from the groupconsisting of natural and synthetic trans-polyisoprene having anunprocessed melt flow index falling within the range of approximately0.2 to 0.8 grams per 10 minutes comprising the steps of:masticating saidpolymer with sufficient shear to heat the polymer until its temperaturereaches a range of from about 90° to 110° C.; coding the polymer to arange of from about 54° to 72° C.; monitoring the temperature of saidpolymer as it is masticated; and discontinuing said masticating stepwhen the temperature of said polymer drops to a range of from about 54°to 72° C.
 10. A process, as set forth in claim 9, wherein saidthermoplastic polymer is selected from the group consisting ofgutta-percha and balata.
 11. A process, as set forth in claim 9,including the additional steps of:heating the polymer during said firststep of masticating until the temperature reaches a range of from about90° to 110° C.; and discontinuing said step of heating when thetemperature reaches said range of about 90° to 110° C.
 12. A process, asset forth in claim 9, wherein mastication is discontinued when the meltflow index exceeds approximately 20 grams per 10 minutes.
 13. A process,as set forth in claim 9, wherein mastication is discontinued when themelt flow index exceeds approximately 50 grams per 10 minutes.
 14. Aprocess, as set forth in claim 9, wherein mastication is discontinuedwhen the melt flow index exceeds approximately 100 grams per 10 minutes.15. A process, as set forth in claim 9, wherein mastication isdiscontinued when the melt flow index exceeds approximately 250 gramsper 10 minutes.
 16. A process, as set forth in claim 9, whereinmastication is discontinued when the melt flow index exceedsapproximately 500 grams per 10 minutes.
 17. A process for theplasticization of a thermoplastic polymer wherein said thermoplasticpolymer is selected from the group consisting of natural and synthetictrans-polyisoprene; having an unprocessed melt flow index falling withinthe range of approximately 0.2 to 0.8 grams per 10 minutes comprisingthe steps of:masticating the polymer with sufficient shear to createheat sufficient to raise the temperature of the polymer to a range offrom about 90° to 110° C.; cooling the polymer to a range of from about54° to 70° C. continuing the mastication for a period of at least about15 hours; and thereafter discontinuing the mastication.
 18. A process,as set forth in claim 17, wherein said thermoplastic polymer is selectedfrom the group consisting of gutta-percha and balata.
 19. A process, asset forth in claim 17, including the additional steps of:heating thepolymer during said first step of masticating until the temperaturereaches a range of from about 90° to 110° C.; and discontinuing saidstep of heating when the temperature reaches said range of about 90° to110° C.
 20. A process, as set forth in claim 17, wherein the masticationis discontinued when the melt flow index exceeds approximately 20 gramsper 10 minutes.
 21. A process, as set forth in claim 17, whereinmastication is discontinued when the melt flow index exceedsapproximately 50 grams per 10 minutes.
 22. A process, as set forth inclaim 17, wherein mastication is discontinued when the melt flow indexexceeds approximately 100 grams per 10 minutes.
 23. A process, as setforth in claim 17, wherein mastication is discontinued when the meltflow index exceeds approximately 250 grams per 10 minutes.
 24. Aprocess, as set forth in claim 17, wherein the mastication is continueduntil the melt flow index is approximately 500 grams per 10 minutes.